Tubular materials have had extensive use as structural components for a variety of apparatuses, for example, bicycle, wheel chair and aircraft frames, and scaffolding. Such tubular materials require high impact and bursting strength, yet are preferably lightweight. Over the years, such tubular structures have been made of any number of different materials, especially metals such as steel and iron, but more recently, in an attempt to reduce weight, aluminum and titanium. Most recently, composite materials have been utilized. Regardless of material choice, however, joining the tubes together is a particularly vexatious problem.
For metal tubular components such as those suitable for bicycle frames, the individual tubular components are secured together by welding, brazing or the like, or bonding into lugs at the tube joints. Steel frames have also been manufactured in "lugless" design, often referred to as fillet-brazed frames. Gussets have also been incorporated into bicycle frames designs. Nonetheless, in each case, welding or brazing is the exclusive method used for joining tubes and gussets, and that method is clearly not employable when composite tubes such as reinforced plastics are used.
All or partial composite tubular structures have been fabricated for, e.g., bicycle frames. As used herein and in the art, a "composite material" is a heterogeneous material which is a combination of two or more chemically distinct and insoluble phases whose properties and structural performance are superior to those of the constituents acting independently; specifically, as used herein, a composite material is created from high-strength fiber reinforcements (the discontinuous or dispersed phase)and an appropriate matrix material, typically, a plastic or polymeric material (the continuous phase). The benefits of using such a material in place of, for example, steel is its greater specific strength (strength-to-weight ratio) than steel.
The most commonly used technique for fabrication of composite material frames is the hand layup method. Sheet-like forms of fiber impregnated with uncured resin (commonly referred to as "prepregs") are applied to a mold of the desired shape or product, and the resin is cured (i.e., the process by which the flexible prepreg is converted from a flexible material in a workable condition into a hardened structural condition).
Besides the use of prepregs, other techniques are known for reinforcing a polymeric matrix. Such techniques include the resin transfer method (RTM). The RTM technique involves use of nonimpregnated fiber sheets or braids which are typically preformed about a mandrel having the configuration of the desired product. The fiber-layered mandrel is placed in a mold and the polymeric resin is injected so that it impregnates the fibers. The use of RTM is described in, e.g., U.S. Pat. No. 4,657,795 issued to Foret for tubular material suitable for a bicycle frame, U.S. Pat. No. 4,828,285 issued to Foret et al. for a bicycle front fork, and U.S. Pat. No. 5,143,669 issued to Mott for construction of a tennis racket.
The bonding of one composite tube to another, however, gives rise to the main difficulty with use of such tubing. Because the fiber-reinforced resin is capable of carrying a tensile stress loading that is many times greater than the resin alone, every seam represents a joint of weakness. To enable transference of stress loadings from fiber to fiber across such seams, the junctures are laminated, i.e., the edge areas of the pieces of prepreg are overlapped. The strength of the composite in such overlapped regions, nonetheless, relies mostly on the adhesive interface in the overlap seam area, and still results in stress concentrations at the junctures and at the exposed fiber ends.
Various approaches to the juncture problem for composite tubing, particularly in the context of bicycle frame construction, have been suggested and are known in the art. For joining tubular components that have already been cured, the components are abutted with each other and an epoxy resin bonding material is applied at the junction formed. Then, one or more prepreg strips are wrapped around the components to tie them together via an adhesive bond that is formed by curing of the prepreg strips. See, e.g., U.S. Pat. No. 4,900,048 issued to Darujinsky; U.S. Pat. No. 5,160,682 issued to Calfee.
Another approach is to form the frame or similar structure using uncured prepreg material. The prepreg materials forming halves of the frame components are laid up in molds along with patches which define the junctions at which the various tubes of the frame are to be incorporated. The entire assemblage is cured and molded at the same time. As a result, the halves of the frame components and the patches all meld into a unified structure. See, e.g., U.S. Pat. Nos. 4,850,607 and 4,902,458 issued to B. J. Trimble where such a technique is applied to the making of bicycle frames. Such frames are known as "monocoque" frames (i.e., "one-piece" frames wherein the frame is molded as a single complete unit). Another approach to increasing the strength of a tubular frame made of composite materials is to eliminate the presence of seams altogether. U.S. Pat. Nos. 5,076,601 and 5,080,385 issued to Duplessis disclose a seamless composite bicycle frame.
Some manufacturers have attempted to bond composite tubing to metal connections to form the tube joints. One approach has involved bonding carbon fiber tubing to cast and machined aluminum alloy connections to form tube joints. Several problems are associated with bonding carbon fiber to aluminum because the two different materials have extremely different structural properties. For example, the coefficients of thermal expansion and the fatigue characteristics of the two materials are substantially different, both of which increase the potential for failure to occur at the connections. Also, the modulus of elasticity of aluminum is substantially lower than that of most composite materials, including commonly used carbon fiber materials. This means that relatively large, bulky connectors, or lugs, are required to provide the needed strength, at the cost of adding weight to the frame being constructed. There is also a difference in the galvanic corrosion potentials between the two materials, sometimes leading to corrosion of the aluminum.
Another approach has been to use an alloy other than aluminum alloy as the metal connector for composite tubing. U.S. Pat. No. 4,902,160 issued to Jeng describes a chromium-molybdenum connector for receiving tubular composites wherein the joint is covered with a coating of the same composite material of which the tubing is made. It is not clear whether such an approach eliminates the problems associated with aluminum connectors.
There are, thus, many practical design problems associated with the use of composite tubular materials as structural components. To date, the art has not adequately responded with the introduction of a tubing that both utilizes the lightweight, high strength-to-weight properties of composites, yet can use conventional, low-cost joining technologies.